Method of forming oxalate coating on chromium alloys



United States Patent METHOD OF FORMING OXALATE COATING ON CHROMIUM ALLOYS Robert C. Gibson, Birmingham, MiclL, assignor to Parker Rust Proof Company, Detroit, Mich., a corporation of Michigan No Drawing. Application October 19, 1950, Serial No. 190,895

2Claims. Cl. 148-614) This invention relates to a method for the coating of chromium and certain chromium alloys for the protection of the metals during mechanical working operations which result in extensive plastic deformation of the metal and also relates to the coated article.

It has been taught that solutions comprising oxalic acid attack certain metal surfaces and produce a coating thereon. Oxalate compositions have been suggested as a base for paints to increase the corrosion resistance of the coating metal and other oxalate compositions have been suggested as so-called drawing compositions. in this latter connection it has been discovered thatjwhere the metal is to be drawn or otherwise subjected to deformation, an oxalate coating on the surface aids materially in protecting the surface against injury.

Not all metals or metal alloys have been found susceptible to the action of the suggested oxalate compositions of the art. As a general rule, metals which resist corrosion will resist the action of oxalate coating compositions. It has been discovered that the presence of iron, for example ferric oxalate, accelerates the rate of attack on some of these resistant metals. For example, my copending application, Serial No. 190,894, now abandoned, filed concurrently herewith discloses that certain solutions comprising'ferric oxalate can be used.

in the coating of nickel and certain nickel alloys which contain substantially no chromium. However; chromium and alloys of chromium in which either nickel or chromium is the major constituent cannot be coated with compositions limitedto oxalic acidand ferric oxalate.

I have now found that the chloride ion has an activating influence toward these latter metals distinct from the accelerating influence of the ferric ion and that improved drawing coatings for chromium and chromium alloys in which either nickel or chromium is the major constituent may be obtained by incorporating chloride ions in an iron oxalate bath provided that certain minimum proportions of chloride and iron are present. 7

I have found that successful coatings are obtained when the bath includes at least about 0.5 percent total iron and includes at least enoughferric iron to increase the solubility of ferrous iron to the extent that the sum of the ferrous and ferric iron ,isequal to at least .5%. The iron can be all ferric iron but cannot be all ferrous and can be in any proportion from 0.5 percent to saturation. A preferred percentage of total iron is l-6 percent. The preferred range of ferrous iron is about 00.8 percent.

In normal practice the iron oxalate is added as ferrous oxalate which is oxidized by oxidizing agents in the bath to ferric oxalate. The preferred oxidizing agent is chlorate but other oxidizing agents may be used. Such oxidizing agents as sodium chlorate and hydrogen peroxide are preferred for the practical reason that they leave no undesirable residue on reduction. It is preferred that the amount of chlorate or other oxidizing agent used be insufiicient to oxidize all the ferrous iron but rather to leave up to about 0.8 percent ferrous ion in the bath.

Satisfactory coatings with the solutions of the present invention may be obtained at coating temperatures in the range of about 85 -l90 F. over a time of about 2-15. minutes. 7 N

The oxalate ions for the coating bath are preferably derived from either ferrous oxalate, ferric oxalate, or oxalic acid. I oxalate salts can be used in combination with an iron salt or an acid to give the equivalent of an iron oxalate solution or the equivalent of a solution comprising iron oxalate and oxalic acid. In any event, however, the

coating solution will include oxalate ions, ferric. ions and preferably some ferrous ions. Oxalic acid maybe added to increase the acidity Without limitation since extremely acid solutions with pH values too low for practical measurement have been used successfully.

Because of the fact that the pH values of the preferred solutions are quite low, I prefer to express the concentration of acid as free acid which as used hereinafter and in the appended claims is defined as the ml. of N/ 10 alkali required to neutralize to a phenolphthalein endpoint 1 ml. of the coating composition to which has been added 10 ml. of 25 percent KF.2H2O. For ex: ample, a free acid of 10 means that a 1 ml. sample of the Coating solution, after the addition of 10 ml. of 25 percent KF. ZHZO, would require 10 ml. of N/ 10 NaOH for neutralization with phenolphthalein indicator. 1 prefer to use a solution wherein the free acid is in the range of about 420 and particularly prefer a free acid content near the high end of the preferred range; that is, about 20.

I have found that there is a definite lower limit of chloride percentage which must be present to secure an effective coating and that this lower critical limit is substantially proportional to the proportion of chromium in the alloy. For example, alloys of chromium containing about 15 percent chromium, the major constituent being nickel, require less chloride than pure chromium or alloys containing relatively higher percentages of chromium than 15 percent. Although the lower limit of chloride may be specified generally for each type of alloy, the exact lower limit is in part determined by the nature of the metal working processes to which the substrate metal has been subjected. For example, a low iron content alloy such as. Inconel containing 14 percent chromium can be coated with compositions of the invention comprising about 0.5 percent chloride ion if the Inconel is annealed. However, Inconel which has been cold rolled and not annealed requires at least about 1.5 percent chloride ion. Thus, the lower limit of chloride for Inconel varies from about 0.5 to about 1.5 percent depending on the nature of the surface of the metal. Thus, a lower limit of about 1 percent may be given for Inconel, it being understood that this lower limit will vary plus or minus about 0.5 percent depending on. I

the pretreatment of the metal.

The chromium alloys of the invention which comprise about 4 percent and may be used as a reference point.

Pure chromium plate requires a minimum chloride ion concentration of about 11 percent and this percentage 7 may be used as a referencelpoint in determining the mini- Patented Dec. 18,1956

It will be apparent, however, that other;

less than about percent iron in the alloy whereas high iron means more than about 10 percent.

If it is desired to coat a metal of the present invention with a coating composition having a chloride proportion near the absolute minimum permissible, it is only a'matter of a few simple tests to establishlthis minimum. In this particular case the only criterion for a successful coating is that the solution react chemically with the surface of the metal and therefore it is merely necessary to raise the chloride percentage to the point where chemical reaction sets in. Thus, the lower limit of chloride ion concentration may be further characterized as that proportion which causes the solution to react chemically with the surface of the metal.

All percentage figures in this specification and in the accompanying claims are percent w./v.; that is, the solu tion is assumed in each instance to have a specific gravity of unity.

The following examples illustrate in a detailed manner the method of the present invention.

Example 1 A series of solutions were made up containing 94 grams ferrous oxalate dihydrate, 54 grams oxalic acid dihydrate, and 9.4 grams sodium chlorate, and sufficient water to make a volume of 2 liters. The chloride content of the baths was varied from percentages as low as 0.15 percent up to saturation. Each of the mixtures was heated to about 160 F. to permit the sodium chlorate to oxidize the ferrous oxalate to ferric oxalate, the amount of sodium chlorate being just suflicient to completely oxidize all the ferrous oxalate without leaving any chlorate residue in the bath. Clean chromium plated panels were immersed in each of the baths at 160 F. for 5 minutes. No coating was obtained where the chloride ion concentration was less than about 11 percent whereas all concentrations of chloride ion above about 11 percent produced a thin gray adherent coating which was valuable in increasing the wear-resistance of the chromium plated surface.

The example was repeated using 4 times the quantity of ferrous oxalate specified above and using a series of solutions with chloride contents varying from 12 to 15 percent. Some of the clean chromium plated panels were immersed in these baths at 130 F. for 15 minutes and others at 150 F. for 5 minutes. In every case an excellent, heavy coating was obtained.

Example 2 Example 1 was repeated using Alloy C (60 Ni, 15 Cr, Fe) as the substrate metal. In the case of Alloy C no coatings were obtained with chloride ion concentrations below about 5 percent, whereas above about 5 percent good coatings were obtained up to saturation of the chloride ion.

This application is a continuation in part of copending application Serial No. 76,181, filed February 12, 1949, which is itself a continuation in part of prior co-pending application Serial No. 782,966, filed October 29, 1947; application Serial No. 76,181 is now Patent 2,577,887, and application Serial No. 782,966 is abandoned.

This application is a continuation in part of co-pending application Serial 76,181, filed February 12, 1949, which is itself a continuation in part of prior co-pending application Serial 782,966, filed October 29, 1947; application Serial 76,181 is now Patent 2,577,887, and application Serial 782,966 is abandoned.

I claim:

1. The method of coating a metal of the class consisting of chromium and chromium-nickel-iron alloys in which the major content is a non-ferrous metal selected from the group consisting of nickel and chromium, said method comprising the step of contacting at a coating temperature the metal surface with an aqueous solution comprising as the essential coating-producing ingredients iron oxalate and the chloride ion wherein ferrous iron is in the range of about 0-0.8 percent, wherein the total iron is at least about 0.5 percent and includes at least enough ferric iron to increase the solubility of ferrous iron so that the sum of the ferrous and ferric iron is equal to at least .5 wherein the solution is at least substantially as acid as that produced by an aqueous solution of iron oxalate alone, and wherein the proportion of chloride ion is at least that proportion which is substantially proportional to the percentage of chromium in the metal with reference points as follows:

1 percent chloride at 15 percent chromium, 11 percent chloride at percent chromium.

2. The method of coating a metal of the class consisting of chromium and chromium-nickel-iron alloys in which the major content is a non-ferrous metal selected from the group consisting of nickel and chromium, said method comprising the step of contacting at a coating temperature the metal surface with an aqueous solution comprising as the essential coating-producing ingredients oxalic acid, iron oxalate, and the chloride ion wherein ferrous iron is in the range of about 00.8 percent, whereing the total iron is at least about 0.5 percent and includes at least enough ferric iron to increase the solubility of ferrous iron so that the sum of the ferrous and ferric iron is equal to at least .5 wherein the solution is at least substantially as acid as that produced by an aqueous solution of iron oxalate alone, wherein the solution contains a proportion of free acid up to about 20, and wherein the proportion of chloride ions is at least that proportion which is substantially proportional to the percentage of chromium in the metal with reference points as follows:

1 percent chloride at 15 percent chromium, 11 percent chloride at 100 percent chromium.

References Cited in the file of this patent UNITED STATES PATENTS 1,895,568 Curtin Jan. 31, 1933 1,895,569 Curtin et a1. Jan. 31, 1933 1,926,846 Giard Sept. 12, 1933 2,060,365 Curtin Nov. 10, 1936 2,105,015 Singer Jan. 11, 1938 2,273,234 Tanner Feb. 17, 1942 2,434,291 Smith Jan. 13, 1948 2,577,887 Gibson Dec. 11, 1951 FOREIGN PATENTS 661,385 Great Britain Nov. 21, 1951 

1. THE METHOD OF COATING A METAL OF THE CLASS CONSISTING OF CHROMIUM AND CHROMIUM-NICKEL-IRON ALLOYS IN WHICH THE MAJOR CONTENT IS A NON-FERROUS METAL SELECTED FROM THE GROUP CONSISTING OF NICKEL AND CHROMIUM, SAID METHOD COMPRISING THE STEP OF CONTACTING AT A COATING TEMPERATURE THE METAL SURFACE WITH AN AQUEOUS SOLUTION COMPRISING AS THE ESSENTIAL COATING-PRODUCING INGREDIENTS IRON OXALATE AND THE CHLORIDE ION WHEREIN FERROUS IRON IS IN THE RANGE OF ABOUT 0-0.8 PERCENT, WHEREIN THE TOTAL IRON IS AT LEAST ABOUT 0.5 PERCENT AND INCLUDES AT LEAST ENOUGH FERRIC IRON TO INCREASE THE SOLUBILITY OF FERROUS IRON SO THAT THE SUM OF THE FERROUS AND FERRIC IRON IS EQUAL TO AT LEAST .5%, WHEREIN THE SOLUTION IS AT LEAST SUBSTANTIALLY AS ACID AS THAT PRODUCED BY AN AQUEOUS SOLUTION OF IRON OXALATE ALONE, AND WHEREIN THE PROPORTION OF CHLORIDE ION IS AT LEAST THAT PROPORTION WHICH IS SUBSTANTIALLY PROPORTIONAL TO THE PERCENTAGE OF CHROMIUM IN THE METAL WITH REFERENCE POINTS AS FOLLOWS: 1 PERCENT CHLORIDE AT 15 PERCENT CHROMIUM, 11 PERCENT CHLORIDE AT 100 PERCENT CHROMIUM. 